1. Field of the Invention
This invention relates to control of exposure duration and more particularly to improved apparatus to minimize the difference in exposure times determined by photosensitive exposure control apparatus when illuminated by substantially constant or flash light.
2. Description of the Prior Art
Many camera shutters automatically vary the time interval of exposure in response to substantially constant and transient (flash) light. The flash light output and mechanical movement of the shutter to initiate the exposure are synchronized. Operation of timing apparatus that establishes a time interval of exposure for the shutter is also synchronized with the mechanical movement of the shutter. Generally, the timing apparatus is coupled to the shutter in such a manner that the time interval of exposure determined thereby begins before the shutter actually opens and ends before the shutter actually closes. That is, when the timing apparatus is actuated to begin the time interval there is a delay before the shutter actually opens. Also, at the end of the time interval as determined by the timing apparatus, there is a delay before the shutter actually closes to terminate exposure. If, however, these delays are equal in duration and scene illumination remains fairly constant, proper exposure should result. When photographing a scene illuminated by flash light, the earliest the time interval can be effectively initiated is when light from the flash light output is reflected onto a photosensor that is part of the timing apparatus. To get maximum use of flash light output, the shutter opens before peak light output occurs. Thus, the time interval cannot ordinarily be initiated soon enough before the shutter opens to offset the delay in closing the shutter after the time interval ends. An additional unwanted exposure is created. Although the additional exposure time is small, it is too long in view of the relatively high intensity, and short duration of the flash light. Exposure error results.
It has been suggested that the exposure error can be minimized if the timing apparatus times out faster during exposures illuminated by flash light than during exposures illuminated by substantially constant light. U.S. Pat. No. 3,200,723 discloses exposure control apparatus that uses an R-C time constant circuit comprising a photosensor and capacitor to establish the time interval during exposures in constant or flash light. The R-C circuit develops a threshold voltage into a trigger circuit that controls the closing of the shutter. For exposures illuminated by flash light, the R-C circuit is altered from the configuration used for exposures illuminated by constant light by connecting a fixed value resistor in the R-C circuit. The resistor causes the capacitor to be initially partially precharged. With an initial precharge, the R-C circuit earlier reaches the threshold voltage of the trigger circuit. The time interval is therefore shorter, and the shutter is closed earlier during exposure illuminated by flash light than during exposures illuminated by constant light.
Because the reflected light is a function of subject distance, the amount of overexposure caused by the difference in initiation of exposure and the delay in closing the shutter is also related to subject distance. If additional correction to the exposure time resulting from reflection of light with subject distance is not considered, exposure error will still result.